Topological Surface States Of Three-dimensional Phononic Crystals

Topological phononics,as an emerging research direction in recent years,is the product of the close combination of condensed matter physics and artificial microstructured acoustics.This concept has being attracted intense attention of researchers.because it can not only enrich the framework of condensed matter physics theoretically,but also possess important research value and application potential.The physical concepts of quantum Hall effect,spin,topological invariant and Berry phase originated from the electronic system is introduced into acoustic systems,resulting in a series of novel phenomena such as acoustic quantum spin Hall effect,acoustic topological insulator,and acoustic pseudospins.Compared to the electronic systems,studying topological phenomena in acoustic systems has its unique advantages,such as Fermi surface-free,more accurate,and easier to control in sample preparation and measurement process.The topological states in phononic crystals have many excellent properties.For example,the topological properties of the phononic bandstructure can ensure topological boundary states with backscattering suppressed acoustic propagation,showing high-efficiency one-way transmitted bypassing defects.However,the research and development of acoustic topological states have just started,and there are still many theoretical questions to solve and novel acoustic materials to explore.This thesis mainly focuses on the study of physics and application potentials of the topological surface states based on two kinds of three-dimensional phononic crystals,including the detail of experimental measurements.Starting from the one-dimensional SSH(Su-Schrieffer-Heeger)model and twodimensional antiphase boundary,we construct and fold the bandstructures of a threedimensional phononic crystal.By manipulating the symmetry of the three-dimensional phononic crystal at the interfac e,the surface states under mirror symmetry and glide symmetry are studied.And by changing the defect layer type,a tunable topological surface state in this three-dimensional phononic crystal is theoretically designed.Secondly,we,for the first time,construct acoustic analogues of three-dimensional topological insulators with com pletely linear(Dirac con e)surface dispersion and onedimensional hinge state using band folding mechanism(spatial increasing then breaking).This progress provides new possibilities for regulating sound waves in various dimensions.Regarding the experimental measurement system,we discuss the details of the experimental setup.A relatively complete research roadmap of three-dimensional acoustic topological crystals has been formed:from sample design to geometry optimization,then 3D printin g preparation,fin ally acoustic measurement.Consequently,we have observed the corresponding phenomenon mentioned above.Our experimental work can serve as an ideal platform to further study topological acoustics.